Coal is the largest source of human-generated mercury emissions in the United States. Coal-fired power plants release about 48 tons of mercury annually, according to EPA data. In contrast, the total amount of mercury in crude oil processed in the U.S. annually is less than five percent of the amount contained in the U.S. coal produced and consumed annually.
Mercury concentrations in crude oil have been reported from as low as <1 ng/g to as high as 50,000 ng/g of oil (see e.g., FIG. 1). Some of the variability observed in crude oil mercury data is due to difficulties encountered in performing the analyses. A wide variety of measurement techniques, including neutron activation and many types of sample preparation systems coupled with detectors as diverse as mass spectrometers, ICP/MS, atomic absorption, and atomic fluorescence have been used to perform these analyses. As such, it can be very difficult to compare mercury analysis results obtained in different laboratories using different analytical techniques. The handling of samples can also significantly affect the measured results. A recent study found that the number of times a sample bottle had been opened could significantly affect the measured concentration.
Although analytical difficulties are responsible for some of the variability in crude oil mercury data, geological factors such as depositional environment and thermal history are more important influence on the concentrations of mercury that are observed in currently produced oils. The mercury concentrations shown in FIG. 1 vary by more than a factor of 1000, which is largely indicative of the wide variety of environments from which these oils originate.
Mercury has come under increasing scrutiny in recent years because its presence in oil creates problems throughout the production, transportation, storage and refining systems. These problems include environmental concerns, contamination of refinery products, catalyst poisoning, corrosion of equipment, health risks for personnel, as well as expenses for disposal of contaminated tank sludge, cleaning of contaminated equipment, shipping restrictions, etc. These issues have led companies to search for methods of reducing the mercury content of crude oil.
Several processes for removing mercury from crude oil have been disclosed in U.S. Pat. No. 6,350,372, U.S. Pat. No. 6,537,443, U.S. Pat. No. 6,685,824, and U.S. Pat. No. 6,806,398 and in an article by Salva (2010). All of these processes essentially involve reacting the crude oil with a sulfur compound to precipitate HgS and then separating the HgS particles from the oil by filtration or another technique for solids removal. In some cases the process requires an additional filtration step prior to contacting the oil with the sulfur compound.
Such processes require the addition of reagent chemicals as well as filtration of the crude oil. The filtration step in particular is problematic because of filter plugging by other components in the crude oil such as waxes and sediments. The HgS will only comprise a small percentage of the total solids removed by filtration. The maintenance of a crude oil filtration unit can become cost prohibitive in terms of manpower, filtration media, and disposal.
The process disclosed in U.S. Pat. No. 8,080,156, in contrast, involves the use of natural gas to strip mercury from the crude oil. However, this process is only effective for the removal of elemental mercury. The efficiency of this process is thus limited by the ratio of elemental mercury to total mercury in the oil, and, in some cases, the efficiency of mercury removal can be very low.
Thus, what are needed in the art are better methods of evaluating and removing mercury from crude hydrocarbons.